Anyone know much about acoustics?
I'm basically building a kind of musical instrument that's a wooden box (thinking birch plywood? - I don't want to spend too much money) with some solenoids and vibrating motors inside that generate noise on various materials' surfaces. The kind of size i picture the box is 30cm x 30cm x 45 cm (1.5ish cm thick)
I'm not particularly knowledgeable about acoustics, would there be a particular optimums size/proportions for the best acoustics? and is there a better (cheapish) material i could use?
I don't think there's anyway to answer this question - how does one define "best acoustics"??
That gets REALLY complicated! You'll just have to experiment! The shape and materials for a (acoustic) guitar or violin were developed empirically over a long period of time. As far as I know, with modern science and computers, we have not come up with any new-better designs.
From the approximate wavelength, I'd say your box is going to resonate around 500Hz or 1kHz... I don't know if a "closed box" resonates at 1/2 wavelength or a full-wavelength.
(1.5ish cm thick)
cm or mm? Again, most wood instruments are quite thin (with the exception of woodwinds & pianos where the resonance of the wood itself isn't that important).
I know there are sites with automated speaker dimension sizing programs for proper ported box size if you input the needed speaker parameters and your desired low frequency cut-off value. But that is probably not relative to your exact application. But one similar requirement is that stiff or thick box material be used to cut down on unwanted resonances. Good hi-fi speaker boxes use very thick wood.
The kind of size i picture the box is 30cm x 30cm x 45 cm (1.5ish cm thick)
From designing recording studios, I can tell you that you do NOT want any two dimensions to be the same, as that will make a pronounced resonance at one frequency. The RATIO of the lengths is what's important to keep different acoustic modes from adding to each other.
Two classical ratios are: 1:1.25:1.6 and 1:1.4:1.9
Since you are building this you can scale the whole things as you see fit.
But DON'T do the 30cm x 30cm for two modes!